1. Field of the Invention
The present invention relates to exposure control mechanisms in photographic cameras, and more particularly to a shutter mechanism including means for synchronizing shutter operation with the output of a chemical flash device.
2. Discussion Related to the Problem
Relatively fast shutter speeds may be used to expose modern, high-speed amateur color film (e.g. ASA 400), thereby reducing the effects of relative camera-subject motion and resulting in sharper pictures. Some popularly priced amateur cameras take advantage of the extra sharpness that can be achieved by having relatively fast (e.g. 1/200 to 1/500 second) shutter speeds for daylight operation. Unfortunately, the performance of low priced amateur cameras employing chemical flash illumination for indoor operation have not similarly benefited from the advent of fast films. One popular format for chemical flash employs a flash firing mechanism comprising a piezoelectric crystal that is struck to generate a pulse of electricity to ignite the contents of the chemical flashlamp. Generally, the piezoelectric crystal is struck at the same time that the shutter is opened, or shortly before (see for example U.S. Pat. No. 3,967,142). FIG. 1 shows a plot of the light intensity produced by such a piezoelectrically ignited flashlamp versus time. At time 0, the crystal is struck. As can be seen from the Figure, it takes approximately 15 milliseconds from the time that the piezoelectric crystal is struck to the point where the output of the flashlamp reaches its peak intensity. If a fast shutter speed is employed (e.g. 2 to 5 milliseconds; 1/500 to 1/200 second) and the shutter is opened immediately upon striking the piezoelectric crystal, the flash intensity will not have approached its peak before the shutter is closed. Most of the flashlight will be wasted. To overcome this problem, most inexpensive prior art cameras are designed to employ a relatively slow shutter speed (e.g. 30 milliseconds) in the flash mode. By using such a slow shutter speed, the shutter is sure to be open when the flashlamp reaches its peak intensity. Although this technique makes full use of the flash illumination available, it sacrifices the sharpness potential in the high speed film. An improvement on this approach to flash synchronization is to delay the opening of the shutter until the flash intensity has approached its peak, then to open the shutter for a relatively short exposure time. The problem faced by the inventors was to provide a simple mechanical shutter mechanism that could be actuated in synchronism with the ignition of a flash device, and after a predetermined delay, would produce a relatively short exposure.